Method of making propeller blades



March 7, 1944, G. T. LAMPTON ETAL METHOD OF MAKNG PROPELLER BLADES Filed Jan. 2, 1941 o/ ///////M0 l/./////// ////0 Patented Mar. 7, 1944 2,343,418 METHOD'UF MAKING PROPELLER BLADES Glen T. Lampton and Harris P. Moyer, Wllliamsport, Pa., assignors, by mesne assignments, to The Aviation Corporation, New York, N. Y., a

corporation of Delaware Application January 2, 1941, Serial No. 372,728

11 Claims.

The invention relates to the manufacture of steel aircraft propellers.

In the operation of modern aircraft the propellers are rotated at a high rate of speed. are generally disposed within the vision of the pilot, and the reflection produced by the outer surfaces of the propeller blades in sunlight is bothersome to the pilot and likely to cause mental fatigue or interfere with proper navigation of the airplane.

One object of the invention is to provide a method of making aircraft propellers, the outer surfaces of which are colored to decrease or from the detailed description.

The invention consists in the several novel features which are hereinafter set forth and are more particularly defined by claims at the conclusion hereof.

In the drawing, Fig. 1 is a vertical section of a furnace and associated apparatus used in carrying out the invention. Fig. 2 is an elevation of the finished blade.

The furnace and apparatus illustrated as an exemplication for carrying out the invention comprise: a foundation or floor 5 which is provided with a well 5; a support for the propeller blades a While they are being treated, which includes a plate 'l having notches therein for receiving and retaining the shanks of a series of blades in suspended and separated relation; a post 8 at the upper end of which plate 'I is secured; and legs 9 fixed to the lower end of post 8 and having their lower ends extending into the well- 5 and supported on the inclined side thereof; a cylindrical bell-shaped inner shell I surrounding the suspended blades and forming a chamber II for heat, gas and air treatment of the blades; an oil-seal between the lower end of the `bell or shell i and the foundation t which consists of a depending flange I2 on the horizontal flange I2l of the shell I 0 and an annular `wel1 i3 in the fonudation which containsoil I4 in which the lower portion of flange I2 is submerged when the shell is lowered; an outer. bell-shaped heater-jacket or casing which includes a refractory lining I5 spaced from the shell I 0 and a metallic shell I6 for said refractory and is vertically movable for heat variation; a series of electric heating elements b which are wound on lthe inside of the refractory I5 and movable therewith; a fan Il centrally mounted in the well 6 for flowing gas and air around the blades in the chamber Il; an electric motor I8 for driving fan Il; a series of inlet pipes I9 for delivering air or gas, as hereinafter described, to the chamber II and connected toa header a delivery pipe 20* leading to header 20; a supply pipe 2i for ammonia gas; valve 22 for controlling the admission of gas to pipe 2| for delivery into chamber I I; an air pipe 23; a valve 24 connected to pipe 2i -for controllingthe delivery of air into chamber Il; a seriesu of outlet pipes 25 connected to a header 26; an .outlet pipe 21 connectedto header 26; and a valve 28 for controlling the outflow of gas or air from chamber The heater jacket, when the furnace is being brought up to the desired temperature, is lowered onto the horizontal flange Illa of shell I 0. The heater jacket I5, i6 with the heating elements therein is adapted to be lifted off the inner shell I!! to provide access to the inner shell and for heat variation of the chamber in the inner shell, by a suitable 'hoisting element 30. The inner shell I 0 is also adapted tobe lifted by any suitable means to provide access to the chamber II for placing propeller blades a into and removing them from chamber II. The

f rates of flow for ammonia and air hereinafter specified are by Way of example and are those suitable for a furnace having a cubic capacity of cubic feet and in which the oil-seai will hold a maximum pressure of 11/2 inches. of water.- It will be understood that variation in the capaci-ty of the furnace requires variation of these Arates of flow.

Before treatment of the blade in accordance with the invention, the surface of the blade is polished to present an even finish, washed with water to remove the effects of the acid or material used in polishing, washed with gasoline or otherwise cle-greased, and wiped dry with a cloth. The blades are then hung into the notches in the supporting plate B, without touching the working surface of the blade with the bare hands so that the outer surfaces of theblades will be thoroughly clean. The furnace is loaded with the blades while the inner shell and heater jacket are raised for access to the blade-support.

The inner shell I is then lowered so its ange I2 is submerged in the oil Il and the chamber I I will be sealed. Valve 22 is opened and ammonia. gas is then delivered at the rate of approximately 100 to 125 cubic feet pen hour to replace the air in chamber Il until the air is reduced to approximately 5%. The heater-Jacket with the heating-elements b therein is lowered around the inner shell I0 and the temperature regulators usually provided are .set `to heat chamber II or the blades therein to approximately 900 F. While bringing the temperature to 900 F. the flow of ammonia gas to chamber Il is reduced to the rate of approximately cubic feet per hour to avoid suillcient increase of pressure to break the oil-seal. This temperature of 900 F. is maintained in chamber II for approximately 5, but not in excess of 15, hours. After the temperature has reached 900 F. the rate of flow ofthe ammonia gas to chamber II is maintained at substantially 20 cubic feet per hour so that the dissociation of the gas will be from 3 to 10%, or the ammonia concentration in the chamber is maintained at 90 to 97%. At the end of said treatment the outer surfaces yof the blades will be nitrided to render them resistant to corrosion and abrasion.-

At the end of this nitriding cycle the heaterjacket is raised until the temperature in chamber I0 drops to about 500 l".y and the flow of ammonia gas is increased to prevent drawing of oil from the oil-seal into chamber II. Next, the heater-jacket is lowered and the heating-elements are actuated and controlled to maintain a stabilized temperature of 585 to 605 F. for about minutes. Next, the flow of gas to chamber II is out off at valve 22. Air under pressure insufficient to blow out the seal, is next admitted under control of valve 24 to chamber II through pipe 2|, header 20 and pipes I! and forced through said chamber at the rate of approximately 100 cubic feet per hour. The temperature during this period is very closely held to 585 to .605 F., which may be done by variably raising or 'I'he delivery of `air lowering the outer shell. to chamber II is continued in this manner until dissociation readings show 80% air present. Next, the oil is drawn from well I3 and cornpressed air under a higher ,pressure is blown through chamber II for about 10 minutes. This treatment of the blade produces a deep blue color in, and uniform finish on, the nitrided surfaces ofthe blade. The heater-jacket and shell I0 are next lifted above the blades and removed so that the blades can be removed from supporting-plate 'I. The blades are then wiped off with a clean cloth saturated with machine oil.

In carrying out the foregoing steps, the outlet valve 28 may be used to control the rate of flow of the ammonia gas and the fan I'I may be'operated to flow the gas and air around all portions of the blades.

When markings for .trade identification or technical data are desired on the outer surface of the blades, such as a trade-mark a', the markings are etched on the outer surface before they are polished and cleaned preparatory to the nitriding and coloring treatment in the chamber 'I I.

The heat treatment of the blades at a temperature of approximately 900 F. in the presence chamber.

of ammonia gas, catalytically breaks down the ammonia into the elements nitrogen and hydrogen and the atomic nitrogen formed acts upon the metal in such a manner as to form nitrides which improve the resistance of the metal to corrosive action and provide it with a hardened surface f orpreventing abrasive action andcorrosion when the propellers are in use. The depth to which this action occurs is a function of the amount of metal in the blades in the chamber II, the duration 4oi its treatment with heat and ammonia gas, and the amount of ammonlain said Theisteel from which the blades are fabricated is usually susceptible to corrosion and abrasion and 'thlspnitriding improves the resistance of the metal'to corrosion and abrasion. The method set forth-contemplates the nitriding of the metal to the depth desired 1n the finished blade and not the additional depth necessary when the machining is doneon the blade after it has been nitrided. The finishing of the blades to their polished and finished condition before nitriding avoids the necessity of nitriding the metal to a sufficient depthfto include the metal machinedoi the blade.

The reduction of temperature from the 900 to 500 rE., the stabilization of the temperature of the blades at a `temperature between 585 and 605u F. before stopping the flow of ammonia gas and the treatment of the blades with air at that temperature, produce a deep blue color on the surface of the blades. 'I'he coloring of the nitrided surface results from the formation of an oxide on the external surfaces of the blade when it is subjected in a heated condition to a reducing temperature. The coloring by this oxide can be controlled within limits by the temperature of the blade. This oxide serves primarily to color the blade but, in addition, augments the non-` corrosive characteristics of the nitrided surface without disturbing its resistance to abrasion or other physical properties. This temperature is critical and the resulting color produced on the surface of the blade is substantially non-reflecting of light, which is a desideratum when the blades are in operation. In practice, it has been found that the color of the blades varies with the temperature from that of light straw to a variety of purples, blacks, light blue and gray, and that at the approximate temperature of 585 to 605 F. a deep .blue color of the blade is produced which is substantially non-reflecting and that the surface is resistant to corrosion and abrasion.

When markings have been etched on the surface of the blades before they are subjected to the coloring treatment, the etched portions are non-reiiecting, legible and permanent.

The invention exemplifies a method of producing propeller blades for aircraft of a surface color such as a deep blue which is substantially nonreflecting of light and which possesses the desired resistance to abrasion and corrosion when the blade is in operation.

The invention also exelmplies a method of permanently marking steel propeller blades in such a way that the etched portions will be non-refleeting and will also be resistant to corrosion and abrasion.

The invention is not to be understood as restricted to the details set forth, since these may be modified Within the scope of the appended claims, without departing from the spirit and scope of the invention.

Having thus described the inventiom'what we claim as' new and desire to secure by Letters Patent-is:

1. 'I'hat improvement in making steel propeller blades for aircraft, which comprises nitriding the outer surfaces of the blade by subjecting it to heat and a suitable gas, and treating the blade after the blade has been nitrided with gas and air at a suitable stabilized temperature and for a suiilcient period to produce a substantially nonreilecting oxidized surface on the nitrided outside of the blade.

2. That improvement in making steel propeller blades for aircraft, which comprises nltriding the outer surface of -the blade by subjecting it 'to heat and a suitable gas, and treating the blade after the blade has been nitrided with gas and air at a stabilized temperature of substantially 585 to 605 F. to produce a substantially nonreilecting oxidized surface on the nitrided outside of the blade.A

3. That improvement in making `steel propeller blades for aircraft, which comprises nitriding the outer surface of the blade by subjecting it to heat and a suitable gas, and treating the blade after the blade has been nitrided with gas and air at a suitable stabilized temperature to produce a substantially non-reflecting deep blue oxidized surface on the nitrided outside of the blade.

. 4. That improvement in making steel propeller blades for aircraft, which comprises nitriding the outer surface of the blade by subjecting it to heat and a suitable gas, and treating the blade with gas and air at a temperature of 585 to 605 F. to produce a substantially non-reflecting oxide surface of deep blue color on the nitrided outside ofthe blade.

5. That improvement in making steel propeller blades for aircraft. which comprises nitriding the outer surface of the blade by subjecting it to heat and a suitable gas, lowering the nitriding temperature of the blade after the blade has been nitrided and stabilizing it at a suitable point in the presence of the gas, and then forcing air around the blades while said temperature is maintained, to produce a substantially non-renecting oxidized surface on the nitrided outside of the blade.

6. 'I'hat improvement in making steel propeller blades for aircraft, which comprises nitriding the outer surface of the blade by subjecting it to heat and a suitable gas, lowering the nitriding temperature of the blade after the blade has been nitrided and stabilizing it at `substantially 585 to 605 F. in the presence of the /gas, and then forcing air around the blades'while they are maintained at said temperature, to produce a substantially non-reflecting oxidized surface on the nitrided outside of the blade.

7. That improvementdn making steel propeller blades for aircraft. which comprises nitriding the outer surface of the blade `by subjecting it to yheat and a suitable gas, lowering the nitriding temperature of the blade after the blade has been nitrided and stabilizing it at substantially 585 to 605 F. in the presence of the gas. and then fbrcing air around the blades until the gas percentage is low, while they .are maintained at said temperature, and then forcing air under higher pressure around the blades while said temperature is maintained, to produce a substantially non-reflecting oxidized surface on the nitrided|` outside of the blade.

8. That improvement in making steel propeller blades for aircraft which comprises nitriding the outer surface of the blade by subjecting it to heat and a suitable gas, lowering the nitriding temperature of the blade after-the blade has been nitrided, stabilizing theitemperature at substantially 585 to 605 F. in the presence of gas for approximately 15 minutes, and forcing air around the blades to displace the gas while said temperature is maintained, to produce a substantially non-reflecting oxidized surface on the nitrided outside of the blade.

9. That improvement in making steel propeller blades for aircraft which comprises nitriding the outer surface of the blade by subjecting it to heat and a suitable gas, lowering the nitriding temperature of the blade after the blade has been nitrided, stabilizing the temperature at substantially 585 to 605 F. in the presence of gas for approximately 15 minutes, forcing air around the blades to displace the gas while said temperature is maintained, and then forcing air under a higher pressure around the blades, to produce a substantially non-reflecting oxidized surface on the nitrided outside of the blade. 4 Y

10. That improvement in making steel propeller blades for aircraft which comprises nitriding the outer surface ofthe blade by subjecting it to heat and asuitable gas, lowering the nitriding temperature of the blade after the blade has been nitrided, stabilizing the temperature at substantially'585 to 605 F. in thepresence of gas for approximately 15 minutes, discontinuing. the ilow of gas to the blades, forcing air around the blades to displace the gas until the gas percentage has been reduced to approximately 20% while said temperature is maintained, and then forcing air under a higher pressure around the blades, to produce a substantially non-reflecting oxidized surface on the nitrided outside of the blade.

11. 'I'hat improvement in making steel propeller blades for aircraft which comprises nitriding temperature of the blade after-the blade has been nitrided. stabilizing the temperature at substantially 585 to 605 F. in the presence oi' gasy for approximately 15 minutes, discontinuing the ilow of gas, forcing air around the blades until the gas percentage is approximately 20%, while said temperature isv maintained, and then forcing air under a higher pressure around the blades for a period of approximately 10 minutes. to produce a substantially non-reflecting oxide surface of deep blue color on the nitrided outside of the blade.

GLEN T. LAMPTON. HARRIS P. NOYER. 

